Since the industrial revolution, a large amount of CO₂ has been emitted into the atmosphere. It prevents thermal radiation from escaping into space, retains more energy within the earth system, and causes warming in the earth surface. It is the well-known "greenhouse effect". The effect has caused severe, widespread, and irreversible influences on climate and human society, such as the rise in sea level, the loss of Arctic sea ice, and more frequent heat waves or cold surges. To limit these influences, the United Nations Framework Convention on Climate Change proposed a temperature target in Paris Agreement in 2015: holding the increase in the global average temperature to well below 2°C above pre-industrial levels and pursuing efforts to limit the temperature increase to 1.5°C above pre-industrial levels.

To realize this target, according to scientific estimation, the CO₂ concentration in the atmosphere must decline in this century and the application of CO₂ removal (CDR) is necessary^1,2. Some questions arise: if CDR is applied, how fast and to what extent does the climate recover? As summarized in IPCC AR6, climate phenomena, such as global mean temperature, hydrological cycle, sea level, and sea ice area, cannot recover to their initial state if CO₂ concentration falls back to pre-industrial level after its increase³. However, the summer rainfall over the Indian monsoon region, demonstrated by the present analysis, is an exception.

Nearly one-sixth of the global population lives in the Indian monsoon region, their socioeconomic well-being is critically dependent on the Indian summer monsoon rainfall (ISMR). ISMR accounts for about 80% of the annual rainfall, and its abnormal variation seriously affect water resources as well as human life. For example, extreme rainstorm hit Northeast India in the summer of 2020, which affected more than 9.6 million people and caused huge economic losses^{4, 5} (Figure 1). In addition, the year-to-year change in rice yield is highly correlated with that of the ISMR⁶. Due to moisture stress, a weak ISMR year generally corresponds to low rice production. Floods due to heavy ISMR events can lead to extensive crop damage. A reliable understanding of ISMR is required to ensure the sustainability of food and water resources in the backdrop of climate change.

A team in the Institute of Atmospheric Physics, Chinese Academy of Sciences, discovered that if CDR is applied, the behavior of ISMR is asymmetric to that when CO₂ concentration increases. The analysis is based on an idealized simulation in which the CO₂ concentration increases by 1% per year until the concentration had quadrupled relative to the pre-industrial level; after it peaks, the CO₂ concentration recovers in a mirrored pathway to the initial concentration. The simulation demonstrated that under the CDR condition, the ISMR declines rapidly and that when CO₂ concentration approaches the pre-industrial level, the Indian monsoon region is drier than that when the simulation starts.

“We find a symmetric CDR pathway may lead to the asymmetric response of ISMR, with anomalous drought relative to CO₂ increasing period.” said Prof. Huang, the team leader.

The authors identified the mechanisms responsible for the asymmetric response of ISMR (Figure 2). The huge thermal inertia of the ocean plays an important role. In the idealized simulation, the ocean continues to absorb heat after the CO₂ peaks: the upper ocean continues to warm for about a decade and then begins to cool; the deep ocean responds more slowly and continues to warm for about 70 years before it cools. As a result, the overall temperature gap between the upper and deep ocean is reduced when the CO₂ decrease relative to when the CO₂ increases. It favors a warmer equatorial central-eastern Pacific (ECEP) when the CO₂ decreases for upwelling prevail in the ECEP. Simultaneously, the warmer ECEP weakens the Walker circulation (rising in the tropical central-eastern Pacific and sinking in the tropical western Pacific), which further reduces the ascendance over India. Meanwhile, the weakened Walker circulation affects the rainfall over the Maritime Continent (e.g., Indonesia) and the tropical western Indian Ocean. The changes in the latent heat release excite the atmospheric wave response (i.e., equatorial Kelvin and Rossby waves), which are unfavorable to moisture transport and convergence over India. Consequently, the processes jointly lead to the asymmetric response of ISMR, resulting in drought risk in India when CDR is applied.

The results indicate that under CDR condition, climate change is highly region dependent. For the Indian monsoon region, CDR may cause a fast decline in rainfall, increase the risk of drought, and may affect water management. This gives a hint that when making climate change policies, one should consider more comprehensive effects, as well as its side effects, of climate mitigation.               

References

1. Sanderson, B. M., O'Neill, B. C. & Tebaldi, C. What would it take to achieve the Paris temperature targets? Res. Lett. 43, 7133-7142 (2016).
2. Xu, Y. & Ramanathan, V. Well below 2 degrees C: Mitigation strategies for avoiding dangerous to catastrophic climate changes. Natl Acad. Sci. USA 114, 10315-10323 (2017).
3. Lee, J.-Y., J. Marotzke, G. Bala, L. Cao, S. Corti, J.P. Dunne, F. Engelbrecht, E. Fischer, J.C. Fyfe, C. Jones, A. Maycock, J. Mutemi, O. Ndiaye, S. Panickal, and T. Zhou, 2021: Future Global Climate: Scenario-Based Projections and Near- Term Information. In Climate Change 2021: The Physical Science Basis. Contribution of Working Group I to the Sixth Assessment Report of the Intergovernmental Panel on Climate Change [Masson-Delmotte, V., P. Zhai, A. Pirani, S.L. Connors, C. Péan, S. Berger, N. Caud, Y. Chen, L. Goldfarb, M.I. Gomis, M. Huang, K. Leitzell, E. Lonnoy, J.B.R. Matthews, T.K. Maycock, T. Waterfield, O. Yelekçi, R. Yu, and B. Zhou (eds.)]. Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA, pp. 553–672.
4. IFRC News. South Asia floods: 9.6 million people swamped as humanitarian crisis deepens. Retrieved from https://reliefweb.int/report/bangladesh/south-asia-floods-96-million-people-swamped-humanitarian-crisis-deepens. Accessed 21 February 2023. (2020).
5. Tang, H., Wang, J., Hu, K., Huang, G., Chowdary, J. S., Wang, Y., et al. Increasing 2020-like boreal summer rainfall extremes over Northeast Indian subcontinent under greenhouse warming. Res. Lett. 49, e2021GL096377 (2022).
6. Wang, B. The Asian Monsoon: agriculture and economy Ch. 18 (Ltd, Chichester, UK, 2006)